The present invention relates to a thermally conductive material and, more particularly, to a grease composition having high thermal conductivity which enables effective dissipation of the heat from electronic parts, and further to a semiconductor device in which the aforesaid thermally conductive grease composition is utilized to effectively remove the heat from electronic parts as a device member, thereby preventing the electronic parts from suffering a serious drop in performance and causing damage due to a rise in their temperature.
Most of electronic parts generate heat while they are used, so that the removal of the generated heat therefrom is necessary for their normal operation. With the intention of removing the heat generated from electronic parts, many means have been proposed. In miniature electronic parts, especially electronic devices provided with integrated circuit elements, thermally conductive materials, such as a thermally conductive grease and a thermally conductive sheet, have been employed as heat removing means (Japanese Tokkai Sho 56-28264 and Japanese Tokkai Sho 61-157587, wherein the term xe2x80x9cTokkaixe2x80x9d means an xe2x80x9cunexamined published patent applicationxe2x80x9d).
In general, an electronic device comprises integrated circuit elements and cap parts for protecting them, and further a thermally conductive material is applied thereto so as to contact directly with both the circuit element and the heat dissipating part, or indirectly therewith via certain materials (See FIG. 1). Accordingly, the heat generated from integrated circuit chips during operation is transmitted to the thermally conductive material and then transferred directly or indirectly to the heat dissipating part, followed by heat release from the heat dissipating part.
With respect to the thermally conductive materials as mentioned above, heat-reducing grease of the type which uses a silicone oil as a base material and a zinc oxide or alumina powder as a thickener is already known (Japanese Tokko Sho 52-33272 and Japanese Tokko Sho 59-52195, wherein the term xe2x80x9cTokkoxe2x80x9d means an xe2x80x9cexamined patent publicationxe2x80x9d). In recent years, aluminum nitride has been developed as a thickener which enables further improvement of thermal conductivity (as disclosed, e.g., in Japanese Tokkai Sho 52-125506).
However, the permissible content of aluminum nitride in a silicone oil used as base oil is restricted within very narrow limits, or to the order of 50-95 weight parts per 100 weight parts of the silicone oil, due to its insufficient oil-holding power, so that the resulting grease composition undergoes no great improvement in thermal conductivity by the use of aluminum nitride instead of other known thickeners though the aluminum nitride itself has high thermal conductivity.
In Japanese Tokkai Sho 56-28264 is disclosed the thermally conductive thixotropic material comprising a liquid organosilicone carrier, silica fiber in an effective amount for prevention of the liquid carrier exudation, and a thermal conductivity-providing powder selected from a group consisting of dendrite-form zinc oxide, thin-leaf aluminum nitride, thin-leaf boron nitride and a mixture of two or more thereof. From this material also, sufficient improvement of thermal conductivity cannot be expected because it is inevitable to reduce the aluminum nitride powder content due to the incorporation of spherical silica fiber as an essential component for enhancement of oil-holding power.
This drawback can be mitigated by using a combination of a particular organopolysiloxane with a spherical aluminum nitride powder having a hexagonal crystal form and grain sizes in a specified range to enable a very large amount of aluminum nitride to be incorporated in the silicone oil (Japanese Tokkai Hei 2-153995). However, aluminum nitride is a very hard material having Mohs hardness of from 7 to 9, and so there are spaces between aluminum nitride grains when they are coarse grains. Therefore, the effect produced by such an increase of the aluminum nitride powder content upon improvement of thermal conductivity is smaller than expected. More specifically, the thermal conductivity attained is of the order of 2.3 W/mK, which is still unsatisfactory.
As a measure to solve this problem, the combined use of fine and coarse aluminum nitride powders has been proposed (Japanese Tokkai Hei 3-14873). While the thermal conductivity is elevated in this case, the resulting composition is too small in consistency (too hard) as grease and has poor dispensation suitability; as a result, it is unsuitable for practical use.
Further, there are proposals such that organopolysil-oxanes of the kind which can hold inorganic fillers in large amounts are employed as base oil and they are combined with at least one inorganic filler selected from the group consisting of ZnO, Al2O3, AlN and Si3N4 (e.g., Japanese Tokkai Hei 2-212556 and Japanese Tokkai Hei 3-162493). However, those combinations are still unsuccessful in attaining a satisfactory level of heat-reducing grease.
The Inventors have already developed a thermally conductive composition utilizing the combination of an aluminum nitride powder with a zinc oxide powder to acquire high thermal conductivity and excellent dispensation suitability, and applied for a patent. Nowadays, however, there is an increasing demand for compositions having higher thermal conductivity than such a composition.
In recent years, on the other hand, electronic devices comprised of unsealed silicon chips have been employed. In a case where a thermally conductive composition utilizing an aluminum nitride powder as a hard filler is applied to the silicon surface of such chips, it has turned out that the hard filler impairs the silicon surface to cause troubles. Such being the case, the demand for compositions causing no damage to the silicon surface has been created lately.
As a result of our intensive studies of thermally conductive compositions having further improved dispensation suitability and thermal conductivity and causing no damage to the surface of silicon chips, it has been found that good results can be obtained by combining a particular base oil with a metallic aluminum powder having an average particle size in the range of 0.5-50 xcexcm, thereby achieving the present invention.
Therefore, a first object of the present invention is to provide a thermally conductive soft grease composition having high thermal conductivity, excellent dispensation suitability and no fear of causing damage to the silicon surface.
A second object of the present invention is to provide a high-performance semiconductor device which is prevented from suffering the overheating of heat-generating electronic parts.
The above-described objects of the present invention are attained with a thermally conductive grease composition comprising (A) 100 parts by weight of a base oil and (B) 500 to 1,200 parts by weight of metallic aluminum powder having the average particle size in the range of 0.5-50 xcexcm, and a semiconductor device utilizing the aforesaid grease composition.
In accordance with the present invention, the grease composition obtained has high thermal conductivity and can be safely applied to a delicate semiconductor material without damaging the surface of the material on contact therewith, because the inorganic filler used can ensure softness in the grease composition. Further, when the present thermally conductive grease composition is applied to heat-generating electronic parts having uncovered silicon surfaces, the resulting semiconductor devices can acquire especially excellent heat-dissipating properties.